Static Spin Susceptibility with Anisotropic SuperconductingGap Parameter in High-T_c Cu Oxides

Abstract

The wave-number dependent BCS spin susceptibility χ (q) with various-type anisotropic superconducting gap parameters Δ (q) is calculated for a two dimensional nearly antiferromagnetic Fermi liquid model with the random-phase-approximation (RPA). The Δ (q) anisotropic but gapful on the Fermi surface gives rise to a significant reduction of χ (q) around q=Q[=(π, π)]. The enhanced χ (q-- Q) below T_c as large as above T_c can be traced to the d(x²-y²)-wave gap parameter or the nodeless anisotropic gap parameter with the specific minimum gap, Δ _min (0.35<2 Δ_min/k_BT_c<0.7), on the Fermi surface. From the experimental findings on the enhanced χ (q-- Q) below T_c and the anisotropic Δ (q) on the Fermi surface for the high-T_c Cu oxides, we conclude that the d(x²-y²)-wave gap parameter is realized but not the other types in the high-T_c superconductivity, except the additional change of the order parameter.